1. Field of the Invention
The present invention relates to a distributed feedback laser diode, and more particularly to a distributed feedback semiconductor laser diode primarily used for optical fiber communications.
2. Background Art
Distributed feedback laser diodes have a structure (or a diffraction grating) in which the refractive index or the gain varies periodically along the traveling direction of the light within the laser. As is known, a distributed feedback laser diode with a uniform diffraction grating having a simple periodic structure oscillates in two longitudinal modes (or at two wavelengths) if both facets of the laser are nonreflective (see, for example, a book entitled “Fundamentals of Semiconductor Laser Engineering”, by Takahiro Numai, Maruzen Co., Ltd., p. 166-171). Therefore, practically, the distributed feedback laser diode is configured such that one facet is nonreflective or low-reflective and the other facet is highly reflective in order to produce a single longitudinal mode oscillation.
Even such a configuration, however, cannot allow every (laser) element to oscillate in a single longitudinal mode. Specifically, whether a laser element can oscillate in a single longitudinal mode depends on the phase of the diffraction grating at each facet of the laser (hereinafter referred to as an “facet phase”). That is, it is theoretically possible to control each facet phase so as to ensure that the laser can oscillate in a single longitudinal mode. However, the period of the diffraction grating is as short as approximately 200 nm and furthermore each facet of the resonator is formed through crystal cleavage in a semiconductor laser manufacturing process, which makes it practically impossible to control each facet phase. Therefore, it happens that even though some lasers can oscillate in a single longitudinal mode, others cannot. This has led to a reduction in the yield of conventional distributed feedback laser diodes.
A diffraction grating structure called the “λ/4 phase shift diffraction grating” has been used to overcome such a problem (see, for example, the above book “Fundamentals of Semiconductor Laser Engineering”). In this structure, both facets of the laser are made nonreflective, allowing the laser to stably oscillate in a single longitudinal mode regardless of the phase of each facet. In this case, the position of the phase shift portion primarily determines the ratio of the optical output from the front facet of the laser to that from the rear facet (hereinafter referred to as “front-to-back ratio”). For example, when the phase shift portion is located at the center portion of the resonator, the front-to-back ratio is 1:1.
It should be noted, however, that though, ideally, both facets of the above laser must be nonreflective, their reflectance may not be able to be set to zero if the laser production accuracy is not sufficiently high. An increase in the reflectance of the facets leads to a reduction in the yield of lasers meeting the requirement of single longitudinal mode oscillation even if they employ a λ/4 phase shift diffraction grating. Therefore, to prevent this from happening, it is necessary to reduce the effective reflectance of each facet. On the other hand, as is known in the art, forming a window structure near an facet can reduce its effective reflectance (see, for example, Katsuyuki Utaka et al., IEEE Journal of Quantum Electronics, vol. QE-20, No. 3, 1984, p. 236-245). Accordingly, this technique may be applied to a λ/4 phase shift diffraction grating to reduce the effective reflectance of the facets.
As described above, each facet of a semiconductor laser resonator is typically formed through crystal cleavage. That is, each semiconductor laser element is cut from a semiconductor substrate by way of crystal cleavage. At that time, the position at which cleavage occurs is not precisely controlled; the variation in the position is approximately ±5 μm to ±20 μm. Such a variation in the cleavage position may cause the phase shift portion to be displaced from its target position (for example, from the center portion of the resonator), which leads to a variation in the front-to-back ratio of the optical output. Since lasers usually utilize only the optical output from the front facet, a variation in the front-to-back ratio implies a variation in the laser efficiency. That is, conventional λ/4 phase shift type semiconductor lasers have a problem in that their laser efficiencies vary due to variations in the cleavage positions. Such variations in the efficiencies reduce the uniformity among the characteristics of the distributed feedback laser diodes, resulting in a reduction in the yield.
In the case of distributed feedback laser diodes having a window structure, on the other hand, the window portion(s) has no diffraction grating formed therein. Therefore, even if a cleavage position is displaced from the desired position, the laser efficiency does not change since the position of the phase shift portion within the resonator (exclusive of the window portion) remains unchanged. However, as described later, lasers having a window structure have a problem in that the light emission direction tends to shift vertically, resulting in a reduction in the coupling efficiency with the optical fiber.
The n-type InP substrate typically has a carrier concentration of 1×1019 cm−3 (or more), which is approximately 10 times higher than the carrier concentrations of the other components of the semiconductor laser. Therefore, due to the plasma effect, the n-type InP substrate has a lower refractive index than the other components. As a result, the laser light is emitted upward since it is affected by the lower refractive index of the n-type InP substrate at the window portion. Furthermore, the n-type InP blocking layer is often set to a high carrier concentration, as compared to the p-type InP cladding layer and the p-type InP blocking layer, which produces a refractive index distribution affecting the light emission direction.
Thus, even though distributed feedback lasers having a window structure do not have the problem of the variations in the laser efficiencies due to variations in the cleavage positions, their emission direction tends to be shifted vertically, resulting in a reduction in the coupling efficiency with the optical fiber.